The present invention relates to a spinal implant set for the dynamic stabilization of the spine according to the preamble of claim 1 and a use thereof.
The present invention allows to realize an elastic connecting rod having different degrees of stiffness along its longitudinal axis and usable for dynamically stabilizing vertebral bodies. Elastic connecting rods are typically used in conjunction with bone screws (pedicle screws) for the dynamic stabilization of the spine. If such a rod is provided with a section of higher stiffness, the possible indications can be substantially extended. Thus, a section of higher stiffness may e.g. be used for the fusion of a vertebral body while a section of lower stiffness may be used for the elastic connection of a neighboring segment.
The “golden standard” in spinal surgery today still consists in the fusion (stiffening) of pathologic vertebral bodies. However, the stiffening often leads to premature degeneration of the segments adjacent to the fusion. Therefore, attempts are being made to provide rods having different degrees of stiffness and to include the neighboring segments in the surgical treatment.
As shown below, the approaches that are known in the art relate to combinations of metal and plastic rods, on one hand, and to plastics rods having variable cross-sections, on the other hand.
The invention according to patent application EP 1 719 468 connects a metal bar to a textile band that is intended to act as an inner traction structure in an outer plastic cushion to form a connected dynamic stabilization.
The invention according to patent specification EP 1 815 812 provides, at the end of a metal rod, a part for receiving a connectable plastic rod.
The invention according to patent application US 2009/0118767 shows a connecting element having variable flexibility along its longitudinal axis. Here the variable flexibility is limited to the area between two anchoring elements, and only the outer body of the connecting element is varied in its stiffness.
The invention according to patent application US 2009/0248083 shows connecting rods having areas of different stiffness along their longitudinal axis which only extend over one motion segment, however, i.e. act between two consecutive fastening screws.
The invention according to patent application WO 97/32533 shows (rigid) connecting rods varying in diameter along their longitudinal axis, which is however limited to the area between two fastening screws.
The invention according to patent application WO 2007/038429 shows a stack of load-bearing elements that should be stiff in the longitudinal direction and elastic in the transversal direction. However, the intended difference in stiffness is only achieved between two respective fastening screws.
The invention according to patent application US 2005/0203513 aims to achieve different degrees of stiffness between respective motion segments or pairs of fastening screws. However, the suggested solution provides no continuous variation over the respective fastening screws so that the connecting element necessarily requires predetermined graduations of the fastening locations, which may result in considerable difficulties for the surgeon in multisegmental applications.
The invention according to patent application US 2007/0191832 suggests a plurality of bodies having different degrees of elasticity between at most two fastening screws and is therefore limited to at most unisegmental treatments.
The invention according to patent application WO 2007/087476 provides different degrees of stiffness between two fastening screws in that the connecting rods have a longitudinal cavity and different external contours. However, the variable cross-sections are limited to unisegmental treatments and due to the cavities are barely suitable for anchoring e.g. elastic rods in bone screws.
The invention according to patent U.S. Pat. No. 7,326,210 suggests connecting elements having elastic areas between the fastening screws that are either suitable for unisegmental applications only or necessarily subject to the problem of the predetermined graduation.
The invention according to patent application WO 2007/089957 also describes variable cross-sections along the rod axis, however in predetermined segments, which requires an individually adapted rod with corresponding segment lengths for each patient. The flattened sides of the rod are orthogonal to the axis of a potential bone screw. Connections of the rod to the bone are not shown.
The invention according to patent application WO 2009/021116 describes variable cross-sections along the rod axis, too, however only between the fastening screws, therefore again requiring a particular rod with corresponding segment lengths for each patient. The suggested dynamic rod system seems not to be suitable for fusions, i.e. for predominantly compressive forces. Furthermore it is doubtful whether the combination of fibers over a molded core can withstand several millions of load cycles without degradation.
The invention according to patent application EP-A-2047812 describes variable cross-sections along the rod axis, however only between the fastening screws, which is again linked to the problem of predetermined segment lengths. Furthermore, the rod is made of metal and cannot be considered as a dynamic fixation system, neither isolated nor on a transitional segment.
The invention according to patent application US 2010/0114165 shows a round rod having a variable cross-section along the rod axis in that the radius decreases in the cranial direction. However, at the cranial end, the rod is not firmly connected to the vertebral body, especially not by using the same bone screws as used further caudally. The invention according to patent specification U.S. Pat. No. 5,217,461 also describes two different diameters of the connecting rod along the rod axis, but does not specify how these different rods are connected to the spine, and even less that one single type bone screw can be used therefore.